We have concentrated, during the major portion of this fiscal year, on elucidation of the physico-chemical basis of the rapid structural changes in the nerve fibers and cells associated with the excitation process. The fact that nerve fibers and cells undergo rapid structural changes when they are excited was discovered and established in this laboratory. Hence, we deem it highly important to direct our efforts toward elucidating how these rapid structural changes are evoked by electric stimulation of excitable tissues. With this aim, we have expanded our analysis of the phenomenon of discontinuous volume phase transition associated with Ca-Na ion- exchange in inanimate anionic gels. We prepared gel samples by free radical copolyinerization of acrylic acid. By determining the degree of gel swelling as a function of mole fraction of the divalent cations in the solution, a phase-diagram was constructed. By recording rapid contractions of thin gel membranes, the factors which determine the rate of transition from a swollen to a shrunken state was clarified. Finally, based on the results of these studies, we have proposed a new, physico-chemical explanation of the excitation process in nerve fibers and cells. In May of this fiscal year, we started to investigate mechanical and thermal signs of the excitation process in the electric organ of the electric eel. It was not difficult for us to demonstrate rapid swelling of the functional units, electroplates, associated with the production of an action potential. Although our study is still in an early stage, we are expecting to gain significant experimental support of our basic idea concerning the mechanism of nerve excitation.